A criterion of paper quality is that it has high brightness and consequently is made from a sufficiently bleached pulp.
Different methods are known for bleaching of cellulose pulp such as addition of chlorine, hypochlorite, oxygen, ozone or hydrogen peroxide. These methods are generally combined in a bleaching process. Lately, environmental problems have attracted more attention and attempts have therefore been made to avoid bleaching with chlorine containing compounds since these, even in small quantities, are considered dangerous to the environment, despite a satisfactory bleaching with these chemicals. Oxygen containing compounds such as hydrogen peroxide, oxygen and ozone have therefore found increased use for bleaching of pulp. These compounds are decomposed to oxygen or substances that are environmentally acceptable. Hydrogen peroxide is preferred over ozone for economic and environmental reasons.
Bacteria are often present when cellulose pulp is bleached, especially when recycled paper is used. The bacteria produce enzymes like peroxidases and catalase that decompose hydrogen peroxide.
Attempts have been made to overcome the degrading action of the enzymes, especially catalase, by overdosing hydrogen peroxide, sometimes 6-8 times the normal dosage. Overdosing the hydrogen peroxide leads to a high dosage of sodium hydroxide and sodium silicate, and accordingly to increased costs.
Inhibition of catalase can also be accomplished by raising the temperature to 60.degree.-90.degree. C. or the pH to 10-14. However, a temperature rise may lead to a faster decomposition of the hydrogen peroxide and increased pH leads to yellowing of the pulp and consequently a need for more intense bleaching or a degradation of the pulp.
It has also been suggested that biocides could be added to kill the microorganisms and prevent the formation of catalase. Biocides are dangerous to the health and unsuitable from an environmental point of view. Moreover, when the microorganisms die, more catalase can come out into the process water.
Metal ions that are present, for example iron, manganese, copper and aluminum ions decompose hydrogen peroxide. To prevent this, complexing agents and sodium silicate are used today. Silicates are inclined to form deposits and the complexing agents are expensive and sometimes dangerous to use. Their influence on the environment is strongly questioned and not completely investigated.
From EP 0 562 835, it is known that small amounts of chlorine dioxide, bromine, chlorine, iodine and ozone can be used as oxidizing, enzyme inhibiting and germicidal agents when bleaching cellulose pulp with hydrogen peroxide. According to example 3 in the description, microorganisms in white water are killed with chlorine dioxide. Chlorine dioxide is a poisonous, explosive, corrosive and awkward substance that easily decomposes and has to be produced on site and kept in a dilute water solution. From an environmental point of view, these kinds of additives are also highly questioned.
U.S. Pat. No. 3,817,828 describes a method in which anionic surface active agents are combined with bacteriacidal substances, e.g. chlorine dioxide, hypochlorite or chlorine water to prevent bacterial growth. The used chlorine compounds are poisonous and dangerous to the environment, and although the method is declared to decrease the use of these, some chlorine compounds will be released to the environment.
G. Galland, E. Bernard, Y. Verac suggest in "Achieving a deinked pulp with high brightness" in Paper Technology, December 1989 different ways to increase the brightness when processing recycled paper by decreasing the hydrogen peroxide consumption. For instance, it is suggested that catalase are removed by eliminating the microorganisms that produce the enzyme or by destroying catalase before bleaching. A sodium hypochlorite treatment may be used to destroy catalase. However, sodium hypochlorite is a substance that is unsuitable for environmental reasons.
In principle the methods given in the two publications involve the use of bleaching agents that are harmful to the environment and are avoided nowadays, in small amounts in order to kill the microorganisms and deactivate the enzyme.
Additions of substances that affect the bleaching with hydrogen peroxide have been suggested. U.S. Pat. No. 3,193,445 describes a method in which the addition of 0.5 to 2.5 moles of acetic anhydride per mole remaining hydrogen peroxide is added in the end of the bleaching process in order to activate the remaining peroxide. Accordingly, the bleaching process of the pulp is prolonged. Derwent's abstract of SU 796 281 describes a method where 0.1-1 (w/w %) oxalate (calculated on dry pulp basis) is added together with hydrogen peroxide under alkaline conditions at 60.degree.-70.degree. C.
However, none of these references mention anything about the possibility to intensify the action of hydrogen peroxide by inhibiting hydrogen peroxide decomposing enzymes like peroxidases and catalases.